In the harvesting of crops it is desired that the grain be separated from other elements or portions of the crop, such as from pod or cob fragments, straw, stalks, and the like. Agricultural combines typically have employed a rotary threshing or separating system for separating the grain from such other crop elements or portions. In general, a rotary threshing or separating system includes one or more rotors, which can extend axially (front to rear) or transversely within the body of the combine, and which are partially or fully surrounded by a perforated concave. The crop material is threshed and separated by the rotation of the rotor within the concave, and the separated grain, together with some particles, such as chaff, dust, straw, and crop residue collectively referred to as material other than grain (MOG), are discharged through the perforations of the concave so as to fall onto a grain bed or pan, or so as to fall directly onto the cleaning system itself.
Cleaning systems further separate the grain from MOG and typically include a fan directing an air flow stream upwardly and rearwardly through one or more fore to aft reciprocating sieves. The air flow stream operates to lift and carry the lighter elements of the MOG towards the rear end of the combine for discharge therefrom. Clean grain, being heavier, and larger pieces of MOG, which are not carried away by the air flow stream, will fall onto a surface of an upper sieve where some or all of the clean grain passes through the upper sieve to a lower, finer sieve. Grain and MOG remaining on the sieve surface are physically separated by the reciprocal action of the sieves as the material moves rearwardly therealong. Any grain and/or MOG remaining on the surface of the upper sieve are discharged at the rear of the combine.
The quantity of clean grain and MOG passing through the sieves is typically controllable, in part, by varying the opening size of the sieves. To this end, sieves include rows of fingers, each row supported on, and rotatably adjustable about, a longitudinal axis. These rows of fingers define laterally extending grain passages between confronting surfaces of adjacent rows of fingers. Rotating the rows through various angular positions increases or decreases the opening size of the passages between the adjacent rows. Thus, material passes through the sieve by falling generally vertically through the spaces between the fingers or by entering the passages between the rows and falling through at the angle defined by the angular position of the rows of fingers as the sieve is reciprocated. As the rows of fingers are rotated more towards a vertical orientation, the opening size of the passages between the rows is increased to allow more crop material to fall through the sieve through the lateral passages. If the opening size of the passages is too large, an increased amount of MOG will be allowed to pass through the lateral passages of the sieve. Conversely, as the rows of fingers are rotated more towards a horizontal orientation, the opening size of the passages between the rows is decreased to allow less crop material to fall through the sieve through the spaces between the fingers and the lateral passages. If the opening size of the passages is too small, less MOG is allowed to pass through the sieve, but less clean grain falls through the sieve as well. Therefore, if the sieve passages are opened too much, increased MOG is allowed therethrough, and if the sieve passages are opened too little, less MOG passes therethrough, but grain throughput is reduced.
Typically the threshing system, the grain pan, a pre sieve, or the like, directs a downward flow of crop material toward the upper sieve. A limited portion of the flow of crop material, typically that portion of the flow directed toward the forward portion of the sieve, includes a higher density of grain and MOG than the portion of the flow of crop material directed further rearwardly on the sieve. Often the cleaning fan system is configured to provide an air flow stream at a significantly higher air flow rate near the forward portion of the sieves coincident with the denser portion of the flow of crop material. For one representative combine cleaning system, for example, the rate of the air flow directed through the forward six to eight inches of the upper sieve has been observed at between about seven to about eight hundred cubic feet per minute (without grain present), which has been observed to be approximately twice the rate of air flow beyond this forward region, for instance, at about three to about four hundred cubic feet per minute observed at twelve to fifteen inches from the front of the sieve.
Ideally while the portion of the flow of crop material including the higher density of grain and MOG is airborne en route to the forward portion of the upper sieve, the flow of air at a significantly higher air flow rate generated by the cleaning fan will be directed therethrough for separating the lighter MOG from the heavier grain such that the lighter MOG will be carried rearwardly over the upper sieve, and the heavier, smaller grain will be allowed to fall onto the upper sieve where it can fall through the spaces between the adjacent fingers of the upper sieve to the lower sieve. Thus, by virtue of the air flow through the airborne flow of crop material, some separation of grain from MOG will occur above the surface of the upper sieve, and some separation will occur on the surface of the upper sieve as a function of the opening size and reciprocation of the upper sieve. That is, under ideal conditions, lighter elements of MOG will be carried by the air flow rearwardly over the upper sieve to be discharged in a desired manner from the combine, heavier elements of MOG will be carried rearwardly by the reciprocating action of the sieves, and grain will fall through the openings of the upper sieve.
When in operation, however, the limited portion of the flow of crop material including the increased density of grain and MOG directed toward the forward portion of the upper sieve having standard spacing between sieve fingers, results in crop material collecting and accumulating on the forward portion of the upper sieve. The accumulation of crop material can build to such an extent as to spill over the forward edge of the upper sieve to the clean grain pan bypassing the lower sieve or into the fan housing. Further, the higher rate air flow stream is unable to pass through the openings of the forward portion of the upper sieve to the extent that the ideal airborne separation above the upper sieve is severely limited or not present at all. As a result the amount of grain cleaned at the forward portion of the upper sieve is severely limited or reduced relative to the ideal situation.
Therefore, what is sought is a combine grain cleaning system which overcomes one or more of the disadvantages or problems set forth above.